Sys.Application.add_init(pageInit);
Sys.Application.add_load(myPageLoad);

function pageInit() {
    alert('init');
}

function myPageLoad() {
    alert('load');
    var pgm = Sys.WebForms.PageRequestManager.getInstance()

    //only bind events on original page load
    //this can be thoguht of as Page.IsPostBack on the server
    if(pgm.get_isInAsyncPostBack() == false) { 
        pgm.add_beginRequest(BeginUpdatePanelRequest);
        pgm.add_endRequest(EndUpdatePanelRequest);
    }
}


function BeginUpdatePanelRequest(sender, args) {
    alert('Beginning Request');
}

function EndUpdatePanelRequest(sender, args) {
    alert('Request Complete');
}